Current day security and economic concerns trumps past history. During the 20th century, the biggest fear of British and American strategists was the unification of German industry with Russian resources. If a single state that comprised of both Germany and Russia were to emerge, such a state would have the power to dominate the entirety of Europe (and Asia) while rendering British and American maritime power completely impotent. China's biggest strength is its manufacturing capability, and it's largest vulnerability is dependence on maritime trade routes to bring in the vast amounts of resources needed to feed its industrial behemoth. On the other hand, Russia control vast expanses of land rich in energy, minerals, and other resources needed for industry, but lack the ability to manufacture the full range of modern day industrial and consumer goods that is own industry and society requires. Thus a perfect economic synergy exists between Russia and China economically. Furthermore, Russia and the Central Asian states form the northern portion of the BRI, which enables China to access not only the resources of these countries, but also allows China to access the markets of Europe, the Middle East, and Africa solely via secure overland trade routes.
Blessed in terms of natural geography, natural barriers protect much of China from invasion. The two historical routes of invasion into China proper were from the North or from the sea. A non-friendly Russia would necessitate the allocation of a substantial amount of resources to defending against invasion from the North, as was the case during the 1970s. Having a friendly Russia along the northern border allows China to focus its attention and resources solely on the maritime invasion route, mainly through building up its navy and long range air force. Likewise, friendly relations with China allows Russia to focus entirely on its Western borders, and frees up Russian military resources from the Eastern military districts.
Chinese should never forget history: the loss of the territories of Northern Manchuria, Mongolia, and Tannu Tuva, and the atrocities committed against Chinese peasants in Manchuria. These events should serve as a reminder of what happens to a nation that has become weak and backwards and powerless to defend itself. However, past historical mistakes should never get in the way of current security and economic needs.
